(RNS) Popes John XXIII and John Paul II will be declared saints on Sunday (April 27), the first-ever double papal canonization. Here’s a quick guide to making saints in the Roman Catholic Church:

There are currently about 10,000 saints on the church’s official roster. The process of canonization is a way that the Catholic Church formally declares that a soul is in heaven and worthy of veneration and emulation by the faithful.

Of the 266 popes, 83 (including John XXIII and John Paul II) have been made saints; almost all of them were canonized in the first millennium of Christianity.

Among the last 10 deceased popes, only Leo XIII (1878-1903), Benedict XV (1914-1922), and Pius XI (1922-1939) are not saints or are not being considered for sainthood. Pope Emeritus Benedict XVI (2005-2013) can’t be considered since he’s still alive. The seven others are saints or are in the canonization pipeline.

The only pope from the church’s first five centuries who was not canonized was Liberius (352­-366), who initially condemned Athanasius, the theologian behind the Nicene Creed that expounds the basics of the Christian faith.

In the early church, saints were sometimes proclaimed by popular acclamation, much as the crowds of mourners tried to do at the funeral of John Paul II in 2005.

There is a five-year waiting period before a cause for canonization can begin, although a pope can waive that requirement, as Benedict XVI did for John Paul II.

In 1234, Pope Gregory IX gave the papacy the final say over whether a dead person who was venerated locally could be officially recognized as a saint by the church.

In 1642, Pope Urban VIII issued decrees that centralized control over the canonization process in Rome. The reforms were partially a response to Protestant reformers who had criticized abuses in the process and the trade in relics associated with saints.

As Rome and the popes took over the canonization process, the number of clerics made saints declined. The popes were more likely to canonize women, who had reputations as healers and miracle workers.

There are three basic steps to formal sainthood: First, a formal inquiry is opened and if a person’s “heroic virtues” are initially confirmed the candidate is called “venerable.” Beatification, usually by the pope, is the second step and the candidate is called “blessed.” Canonization is the third and final step, when a candidate is formally declared a saint.

The sainthood process remained largely unchanged until John Paul II approved revisions in 1983; the biggest change was to eliminate the “devil’s advocate,” who was charged with trying to poke holes in a person’s sanctity.

Two miracles are generally required for canonization, although the pope can dispense with that requirement, as Francis is doing in canonizing John XXIII, who was credited with just one miracle.

Nearly all miracles are unexplained medical cures, and they are verified by a panel of medical and scientific experts — not all of them Catholic — who must affirm that there was no possible natural cause for the cure. The cures are usually instantaneous.

Martyrs who are killed for their faith can be declared saints without any proof of miracles.

John Paul II is being canonized just nine years after his death, the fastest a pontiff has ever been made a saint.

Others have been made saints more quickly: St. Anthony of Padua was canonized in 1232, less than a year after his death, and St. Francis of Assisi was canonized 20 months after his death. Joan of Arc was burned at the stake in 1431 but was not canonized until 1920. Other popular saints, including St. Patrick, were canonized before their was an official process.

Pope Francis has sought to rein in the costs of a canonization, which can run up to $1 million for the entire process.

Sources: “Understanding the Canonization Process,” by Christopher Bellitto; “John XXIII: The Medicine of Mercy,” by Massimo Faggioli; the EWTN website.